Exemplary of patents showing use of EVA resins to produce films suitable for use in packaging are U.S. Pats. No. 3,741,253; 4,064,296; 4,457,960; 4,247,584; and 4,352,844.
A film known from U.S. Pat. No. 3,741,253 to Brax et al comprises a core layer of a vinylidene chloride copolymer (saran) between a layer of ethylene-vinyl acetate copolymer and layer of a cross-linked ethylene-vinyl acetate copolymer. Ethylene-vinyl acetate copolymer (EVA) has some improved properties over the previously used polyethylene. Vinylidene chloride copolymers are known barrier materials to fluids such as oxygen.
As disclosed in U.S. Pat. No. 4,064,296 to Bornstein et al the core layer may also be a hydrolyzed ethylene-vinyl acetate copolmyer (EVOH). It has similar oyxgen barrier properties as vinylidene chloride copolymers and offers the advantage that it may be irradiated without discoloration, which is further discussed below.
Blends of linear low density polyethylene and ethylene-vinyl acetate copolymer in oriented barrier films are disclosed in U.S. Pat. No. 4,457,960 to Newsome, which claims an oriented multiple layer polymeric film, comprising (a) a first barrier layer, said first layer having two opposing surfaces; (b) a second layer adhered to one said surface, said second layer being 10% to 90% linear low density polyethylene and 90% to 10% ethylene vinyl acetate; and (c) a third layer adhered to the other said surface, the composition of said third layer being selected from the group consisting of (i) ethylene-vinyl acetate, and (ii) blends of 10% to 90% linear low density polyethylene with 90% to 10% ethylene-vinyl acetate.
U.S. Pat. No. 4,247,584 to Widiger et al discloses a bag for packaging meat, comprising a laminated film fabricated into a bag by heat sealing together confronting portions of an outer layer, said film being a laminate comprised of: a molecularly oriented base layer of a vinylidene chloridevinyl chloride copolymer containing about 90 to 70 weight percent of a molecularly oriented outer layer of an EVA blend, said blend being comprised of about 10 to 90 weight percent of a low EVA random compolymer containing about 2 to 12 percent of vinyl acetate and having a melt index of about 0.2 to 10, and about 90 to 10 weight percent of a high EVA random copolymer containing about 8 to 30 percent of vinyl acetate and having a melt index of about 0.2 to 5, the percent of vinyl acetate in said low copolymer being below that in said high copolymer, and said blend having a weight average vinyl acetate content of about 4 to 15 percent. This patent states that the EVA blends utilized require a suitable amount of a relatively low VA copolymer to provide sufficient crystallinity for strength, and a suitable amount of the relatively high VA copolymer to afford desirable processing characteristics (flow, lubricity). Also is stated that the melt index of the blend must be sufficiently high for molecular orientation, and yet low enough to prevent excessive flow at elevated temperatures, which will cause shrink holes to be produced; the indices of the components should also be relatively close to one another to ensure compatibility, and good optical properties. Also is stated that the melting point of the blend can also be a significant factor, since too low a value will reduce toughness, and too high a value will make the orientation temperature more critical to satisfactory results; generally, the melting point of the blend should range from about 97.degree. to 110.degree. C.
U.S. Pat. No. 4,352,844 to Bornstein discloses a multilayer, heat-shrinkable packaging film having a first EVA surface sealing layer, a second EVA layer adjacent thereto, and a barrier layer adjacent the second EVA layer. The surface sealing layer comprises an EVA that has a higher melting temperature than the second EVA layer. Also, the second EVA layer is at least 1.5 times the thickness of the first EVA layer, and controls the shrink temperature of the entire multi-layer film. The film can be made into bags faster than prior art films can due to the surface EVA layer having less tackiness than the interior EVA layer.
It is an object of this invention to provide a packaging film and bags made therefrom having excellent or improved orientation characteristics over those of the materials used in the past. This means that the orientation speed during processing should be faster.
It is a further object of this invention to provide a process for manufacturing heat-shrinkable, multi-layer packaging film, wherein the orientation can be conducted at a much faster speed than was heretofore possible. Accordingly, the advantage of the invention is that factory production will increase without any concomitant increase in machinery and/or the labor force.